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Unformatted text preview: Types of volcanic features
• Flood basalts: regional lava ﬂows that come from
ﬁssures (long, linear cracks in the earth's surface);
thick accumulations build lava plateaus Fig. 9.9 Lecture 8: 1 Types of volcanic features
Iceland Lecture 8: 2 Types of volcanic
features Types of volcanic features Kraﬂa splatter cones,
Photo: R.W. Schlische Kraﬂa
Splatter cones along
ﬁssure eruption Lecture 8: 3 Lecture 8: 4 Types of volcanic features
• Flood basalts / ﬁssure eruptions Laki crater row, southern Iceland
Photo: Y. Arthus-Bertrand Types of volcanic features
• Flood basalts / ﬁssure eruptions Fissure on Kilauea’s western rift
Photo: M. Levy Columbia River basalt and basalt of Snake River Plain are ﬂood basalts. Lecture 8: 5 Lecture 8: 6 Types of volcanic features
• Ancient ﬂood basalts of Columbia River region Products of eruptions
• Gases: CO2, H2O, SO2, H2S, HCl, N2 Photo: P.L. Kresan
Lecture 8: 7 Fig. 9.18 Photo: P.L. Kresan
Lecture 8: 8 Products of eruptions Products of eruptions • Pyroclastic (fragmental) material, e.g., ash, bombs • Pyroclastic (fragmental) material, e.g., ash, bombs Volcanic
Source Paricutin cinder cone eruption in 1944, Mexico
Photo: E. Tad Nichols
Lecture 8: 9 Lecture 8: 10 Products of eruptions Products of eruptions Lava
Kilauea Photo: USGS
Lecture 8: 11 Lecture 8: 12 Types of basaltic lava Products of eruptions • Pahoehoe lava: billowy and ropelike (hotter) “Fossil” lava “falls”, Kilauea
Kilauea Photo: R.W. Schlische
Lecture 8: 13 Types of basaltic lava Photo: USGS
Lecture 8: 14 Types of basaltic lava
• Pillow lava: spheroidal masses produced by underwater
eruptions • Aa: rough surface composed of blocks and fragments
(cooler) Active aa ﬂow,
Kilauea Fig. 6.23
Lecture 8: 15 Photo: W.H.O.I.
Lecture 8: 16 Plate tectonics / viscosity Viscosity of lava
• Viscosity: resistance to
• Viscosity increases as
increases Continental rifting /
continental hotspots • Basalt -- low viscosity;
flows easily Convergent boundaries • Rhyolite -- high
viscosity; does not
flow easily Divergent boundaries & oceanic
hot spots Lecture 8: 17 Lecture 8: 18 Plate tectonics / viscosity
Continental rifting /
continental hotspots Convergent boundaries Volcanic eruptions
• George Julius Poulett Scrope (17971896, England): “…the main agent in
all these stupendous phenomena
consists unquestionably in the
expansive forces of some elastic
aëriform ﬂuid struggling to escape
from the interior of a subterranean
body of lava.”—Considerations on
Volcanoes, 1825. Divergent boundaries & oceanic
hot spots Fig. 9.2
Lecture 8: 19 Lecture 8: 20 Volcanic eruptions Volcanic eruptions
• Magma contains dissolved gases • Importance of gas is indicated by
pyroclastic rocks • Rising magma --> pressure reduction
• --> gas release
• --> eruption
• Soda-bottle & beer-pouring analogy Pumice, scoria--volcanic rock containing
numerous cavities ﬁlled with gas
Ash--shards of volcanic glass formed in
explosion Lecture 8: 21 Lecture 8: 22 Volcanic eruptions Volcanic eruptions • Basaltic lava: gas escapes readily-->quiet eruptions
Fig. 9.2 • Viscous lavas: gas escape is impeded -->
buildup in gas pressure-->explosive
Fig. 9.5 1991 Eruption of
Mt. Unzen, Japan.
Photo: AP Lecture 8: 23 Kilauea Volcano, Hawaii. Photos: USGS Lecture 8: 24 Types of volcanoes Review Questions
8-1. A. True / B. False: Volcanism was responsible for producing the Earth's
atmosphere. Fig. 9.12
Largest; gentlest slopes;
very fluid lava 8-2. A. True / B. False: A caldera is larger than a crater.
8-3. A. True / B. False: Viscosity increases as the silica content of lavas increases.
8-4. Which type of lava has the lowest viscosity?
C. rhyolitic Viscous lava &
pyroclastics 8-5. A. True / B. False: Explosive volcanic eruptions are associated with highviscosity lava because gases cannot readily escape, leading to a build-up in
pressure and eventual explosion. Smallest; steepest
slopes; pyroclastics 8-6. Which type of lava has a relatively smooth, ropy or billowy surface?
8-7. Which type of lava has a very rough surface?
C. pillow Lecture 8: 25 Review Questions
8-8. Which type of lava forms under water?
8-9. A. True / B. False: Composite volcanoes are commonly found at hot spots.
8-10. A. True / B. False: Eruptions associated with composite volcanoes tend to be
much less destructive than eruptions associated with shield volcanoes.
8-11. A. True / B. False: Rhyolitic magmas are associated with lava domes.
8-12. A. True / B. False: Shield volcanoes have the gentlest slopes because the very
ﬂuid basaltic lavas can ﬂow large distances before cooling.
8-13. Of the three primary forms of subaerial volcanoes, ____________ are the largest
B. cinder cones C. shield volcanoes
8-14. Of the three primary forms of subaerial volcanoes, ____________ are the
smallest in area.
B. cinder cones C. shield volcanoes
Lecture 8: 27 Lecture 8: 26 Review Questions
8-15. Explosive or voluminous eruptions may cause the volcano to collapse on the
ﬂoor of the (now empty) magma chamber, producing a broad depression termed a
8-16. The smoky cloud that rises from the vent of an actively erupting volcano is
composed of ____________.
A. smoke from wildfires inside the volcanic chimney
B. smoke from wildfires on the outer slopes of the volcano that become funneled
into the crater
C. fine volcaniclastic debris (ash) suspended in the air
D. a continuous fountain of dark, basaltic lava
8-17. Rhyolitic lavas ____________.
A. do not flow as far from the vent as basaltic lavas do
B. cool much more slowly than basaltic lavas do
C. are associated with volcanoes that almost never emit pyroclastic debris
8-18. Pillow lavas are associated with ____________.
A. continental rhyolitic eruptions
B. continental basaltic eruptions
C. submarine rhyolitic eruptions
D. submarine basaltic eruptions
Lecture 8: 28 ...
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- Fall '11